The present invention relates generally to adaptive equalizers and particularly concerns a technique for negating the effects of DC offsets in the operation of such equalizers.
It is known to incorporate reference signals in a television broadcast for various purposes including, for example, synchronizing the reproduction of the displayed image and operating an adaptive transversal filter for canceling ghost signals. To provide improved noise immunity, the reference signal may, for example, comprise a pseudo noise (PN) sequence. In the ghost canceling application, ghost signals introduced into the broadcast are canceled by using the received reference signal to identify the presence of ghost signals which are then replicated by a transversal filter and subtracted from the received broadcast. More particularly, an error signal is developed in the receiver by subtracting a stored replica of the reference signal from the received reference signal, which may be accompanied by one or more ghost components. The error signal, which therefore represents the existence of ghost components, is then used in a feedback loop to control a adaptive transversal filter for generating replica ghost signals which are subtracted from the received signal to cancel the ghost signals.
Quite often, during either or both the transmission or demodulation process, the received reference signal can acquire a DC offset which appears as a relatively constant component in the error signal fed back to the transversal filter. This constant component of the error signal prevents proper operation of the transversal filter which therefore can no longer generate correct ghost replica signals. The susceptibility of the reference signal to DC offsets thus severely limits its usefulness in controlling the transversal filter for generating ghost replicas.
It is therefore a basic object of the present invention to provide a transmission system in which a reference signal is used for facilitating the accurate generation of replica ghost signals in an adaptive transversal filter.
It is a more specific object of the invention to provide a system for negating the effects of DC offsets acquired by a reference signal during transmission and/or reception to facilitate the use thereof for controlling the generation of replica ghost signals in an adaptive transversal filter.
These and other objects are achieved according to the invention by alternately transmitting the reference signal in inverted and non-inverted form and reinverting the received reference signal transmitted in inverted form prior to generating the error signal, thereby negating the effects of DC offsets acquired by the reference signal during transmission and reception.